It’s nice for people to believe in themselves. Being optimistic is a good thing.
However, being overly optimistic will lead people to question everything else about you as well.
If you think you can increase your sales by 20% and can give some empirical evidence to back it up, people will believe you and and want to help.
If you think you can increase your sales by 500% your believability level goes down dramatically and it will encourage others to doubt anything else you say.
Optimism is good, overly optimistic can destroy your reputation.
Have a great day!